Low voltage 14 mev. neutron source



fE. R. GRAVES ET AL LOW VOLTAGE 14 MEV. NEUTRON SOURCE sept.- 29, 1959 5Sheets-Sheet 1 Filed April 7, 1954 IN V EN TORS E /izabef/z GravesW/TNESSES.'

Sept. Z9, 1959 E. R. GRAVES ET AL 2,906,903

Low VOLTAGE 14 MEV. NEUTRON SOURCE Filed April 7, 1954 3 Sheets-Sheet 2l 27 z 301/ i;

l F'lg. 2

\ 4 s W/T/VESSES: El' b M N/ENTORS: 1 /za e raves Mgg, BY Rober! /V.L/ff/e, Jr.

Sept. 29, 1959 E. R. GRAVES :TAL 2,906,903 Low VOLTAGE 14 MEV. NEUTRONSOURCE Filed April 7, 1954 3 Sheets-Sheet 3 SPARK/IVG POTENT/AL (KV) Fig. 3

D0 Source *l To /0 00 Source To 34 Flg- 4 532V *27033854 Source i ToGround F ig. 5

` W/'VESSES.' INVENTORS:

E//abe//z l?. Graves www BY Raben N. iff/e, ./f.

nited States Patent LOW VOLTAGE 14 MEV. NEUTRON SOURCE Elizabeth R.Graves, Los Alamos, N. Mex., and Robert N Little, Jr., Fort Worth, Tex.,assignors to the United States of America as represented by the UnitedStates Atomic Energy Commission Application April 7, 1954, Serial No.421,714

9 Claims. (Cl. 313-61) The present invention relates to particleaccelerators, and more particularly to devices in which positive ionsare accelerated and produce neutrons upon impact with a high potentialtarget.

It is well recognized in the construction of neutron producing ionaccelerators that the production of high neutron yields necessitateshigh ion beam currents, requiring high ion densities, and, therefore,relatively high gas pressures for their production. However, in usinggas pressures which are higher than conventional ion acceleratorpressures, the problem of sustaining high accelerating potentialswithout troublesome discharges, i.e., gaseous breakdown, becomesincreasingly important. Therefore, neutron producing ion accelerators ofthe prior art usually incorporate conventional long accelerating tubesand gaps using low gas pressures with resulting beam currents of about100 microamperes and neutron yields of 105 per second. Furthermore, inthe neutron producing devices of the prior art, the ionizing chamber andaccelerating chamber are in many cases operated at different gaspressures. In these devices the ionizing chamber, which is maintained ata higher gas pressure than the accelerating chamber, is separated fromthe ion accelerating chamber by some device, such as a' very smallaperture, which will maintain some pressure difference but will stillallow ion passage. These separate chambers are required because of theuse of conventional long ion accelerating tubes. The accelerating tubesare long in order to focus the beam of ions, and low pressures are usedin order to prevent voltage breakdown as well as to reduce ion beamscattering. Such devices are diicult to use in a portable fashion,because ofv their size and fragile construction.

The present invention is directed toward attaining substantially higherneutron yields than the devices of the prior art, and at the same timereducing the overall dimensions of the neutron source. The presentinvention utilizes the principle of comparatively high gas pressures,high ion densities, and high ion beam currents, Without the usuallyencountered problems of high voltage discharge, and pressure differencemaintenance.

It is the object of the present invention to provide a small, compact,sturdy, portable, low voltage, neutron source capable of yielding at-least 108 neutrons per second, for use in well logging operations andin laboratory experiments requiring periodic use in a variety oflocations.

A further object of the present invention is to provide such a portable,high yield neutron source by ionizing deuterium gas and accelerating theresulting ions against a target containing a hydrogen isotope whereinthe nucleus contains one or more neutrons.

A still further object of the present invention is to provide such aportable, high yield neutron source using ionized deuterium gas, whichsource has a short ion accelerating gap and is not susceptible togaseous discharge of the high accelerating potential.

A still further object of the present invention is to Ilice" providesuch a portable, high yield neutron source using ionized deuterium gasand having a short accelerating gap, which source is contained withinone chamber, said chamber being maintained at a uniform gas pressurethroughout.

A still further object of the present invention is to provide such aportable, high yield neutron source using ionized deuterium gas, andhaving a short accelerating gap and a uniform gas pressure, whichneutron source can be filled with deuterium gas and sealed off toeliminate burdensome gas inlet lines, deuterium gas supplies, and vacuumpumps.

A still further object of the present invention is to provide aportable, high yield neutron source using ionized deuterium gas, a shortaccelerating gap and uniform pressure, which neutron source may besealed off and which produces neutrons according to one of the followingreactions:

where the neutrons produced have energies of 14 meV. and 2.5 mev.respectively.

Other objects relating to the organization, arrangement and cooperationbetween the constituting elements will be apparent from the descriptionand drawings. In the drawings, herebypmade a part of the specification:

Figure l is a cross sectional view of the device showing the generalrelation and location of various parts,

Figure 2 is an enlarged sectional view of the ionization andaccelerating chambers, showing in detail the interior components of thedevice,

Figure 3 is a graph showing, in general, the relationship between thesparking voltage and the product of pressure and distance,

Figure 4 shows schematically the electrical components and theirconnections to the apparatus, and

Figure 5 is an enlarged cross sectional view of a portion of theionization chamber showing a modification.

Referring now to the drawings, Figure l shows the assembly and generalarrangement of the elements constituting the neutron source, whichconsists of a cylindrical electrically grounded case 10 having hangedends. This case is preferably fabricated of stainless steel, therebysimplifying the problem of outgassing hereinafter discussed in moredetail.

A high frequency type electrical insulator 11, preferably made of avitreous material, is sealed in a vacuum tight fashion to ilange member12. The insulator has a cylindrical longitudinal aperture through whichthe target support mechanism is inserted. The ange member 12 is sealedby means of an O ring or other similar sealing device to the upperflanged portion of the case 10. The flange member 12 is locatedapproximately at the middle of the longitudinal length of insulator 11,so that each end of the insulator is about equidistant from the point ofcontact with the grounded case 10. A conducting tube 13 is locatedwithin the central aperture of the insulator 11, and extends from bothends of the insulator 11. The upper extending portion of tube 13 issealed to the insulator 11 around the periphery of the tube, and has anoutwardly extending flange at its extreme upper end. A sealing tube 14,having a flange at the lower end, is sealed by means of an O ring orother similar device to the upper flange of tube 13. The lower extendingportion of the tube 13 is threaded.

Concentric with, spaced from, and extending from both upper and lowerends of tube 13, is target support tube 15. The upper extending portionof support tube 15 is sealed by means of an electrical insulating andvacuum tight seal 16 to the upper portion of sealing tube 14. 'I'hesupport tube 15 is kept concentric with and spaced from the tube 13 bymeans of the seal 16 and an insulating spacer 17 (see Figure 2), thelatter being located at the lower extremity of tube 13. Thus, tube 13and tube 15 are insulated from each other and, therefore, are capable oftransmitting `electrical potentials of different magnitude.

Referring now to Figure 2, a target backing element 18 is fixed to thelower extending extremity of target support tube 15, and comprises a cupshaped member with the periphery of the cup forming walls electricallyconnected and vacuum sealed, by means of solder or similar means, to thesupport tube 15. Thus, the interior of tube 15 is completely sealed offfrom the area adjacent its lower extremity, but lis open to theY areaadjacent its upper extending portion. Extending into the open upperportion is a cooling tube or duct 19 which extends through the entirelength of target support tube 15, and,V which terminates a Vshortdistance from the target backing element 18. The cooling duct 19 ismaintained at the same potential as the target 2), but is electricallyinsulated from a high pressure air source (not shown). The air passingthrough duct 19 functions to cool the target backing element 18.

The target 20 consists of a tungsten backing having one surface ofzirconium in which a hydrogen isotope containing one or more neutronsi.e., deuterium or tritium, preferably the latter, has been absorbed.The target backing element 18 and the back of the tungsten target, i.e.,the surface opposite the tritium-containing surface, are each groundtiat, and then lapped together to provide a maximum heat transfersurface between the target'and the target backing element 18. It isessential that the target be maintained below the temperature at whichtritium is released from the zirconium, otherwise the target life willbe greatly reduced. In practice it is desirable to use a given tritiumtarget for an indefinite length of time with negligible loss of yieldfrom the D-T reaction in the target. If the target becomes too hot,tritium gas will be released and lost. This difficulty is avoided byoperating the target below some critical temperature, hence thenecessity for providing the cooling air duct 19. The highest temperaturefor safe operation of this device is of the order of 300 C.

The target 20 is firmly held against the target backing element 18 bymeans of a washer 21. This washer is fixed to target backing element 18by means of bolts, or other well-known means, and its central opening isconcentric with the periphery of the target.

Since the target 20, while under bombardment by accelerated deuteriumlions, emits secondary electrons, a secondary electron suppressor 22 isprovided. The suppressor 22, in the form of an open ended envelope isattached to the threaded lower extremity of tube 13. The upper peripheryof the suppressor is formed so that upon assembly the envelope is rmlyheld against the bottom of the insulator 11, thereby reducing thedistance from the grounded case to the elements maintained at highvoltage. The lower extremity of the suppressor envelope 22 has aninwardly extending opening, the opening being the same size as, andcoincident with, the exposed area of the target 20. The envelope 22 hasa hemispherical lower end surface which is concentric withthehemispherical anode hereinafter described.

The envelope being connected to the tube 13 which is insulated from thetarget support tube by means of seal 16 and insulator 17, can bemaintained at a slightly greater negative potential with respect to thetarget. Thus, secondary electrons emitted from the target will berepelled by this negative potential and thereby induced to return to thelower negative potential area of the target 20. The suppressor 22 ispreferably fabricated from stainless steel or similar material which hasthe characteristic of reduced tield emission.

The ion source and focusing assembly consists of an anode support plate23 which is attached and sealed to the lower flanges of case 10. Theplate 23 has a centr aperture'through which the anode assembly extends.`

The anode assembly consists of a graphite cylindrical anode 24 having ahemispherical cavity at one end, and a centrally located circularaperture concentric with the axis of the hemispherical cavity extendingthe remaining length of this anode. The anode 24 has peripherallongitudinal grooves so that gas may be passed along its periphery. Apump lead 25 is connected through the case 10 adjacent the lower llangethereof, thereby connecting the pump lead to the peripheral grooves ofanode 24. Thus, gas located in the area adjacent the envelope 22 andtarget 20 may be removed by means of a vacuum pump attached to pump-lead25. The envelope 22 and the interior surface of case 10 combined withthe hemispherical cavity in anode 24, and the upper surface of anode 26,form a U shaped volume having a virtually constant thickness. Thus, thedistance from the high electrical potential on envelope 22 and theexposed sur-4 face of target 20 is virtually Yconstant in allperpendicular directions toward grounded elements 10, 24 and 26.

The anode 24 is xed to support plate 23 so that they central aperture ofanode 24 and the aperture of support plate 23 provide a continuouspassage through both elements. Supported on the opposite side of plate23 and' extending through the continuous central aperture of ele-- ments23 and 24 is a graphite focusing anode 26. Anode 26 consists of acircular plate with a centrally located:

The central aperture of anode 26 consistsof an upwardly ex' tendingconical aperture which is coaxially truncated by` aperture and anoutwardly extending circular flange.

a circular aperture extending the remaining distance through thecircular plate of the anode 26. The circular aperture which faces thetarget is smaller than the diameter of the target so that stray ioncurrent to the envelope` 22 is minimized. The periphery of the outwardlyextending tlange has a downwardly extending radially taperingv edgewhich terminates in an annular lower surface.

Spaced from and concentric with the outer periphery ot'V the anges ofanode 26 is a circular graphite anode 27.

The upper peripheral edge of anode 27 has an upwardly extending radiallytapering edge similar to anode 26, andA Thus, the tapering edges ofanodes 26 and 27 define a cylindrical areav also terminates in anannular surface.

having rounded edges.

Graphite supportingV and spacing tubes 28 separate',

anode 26 and anode 27. Bolts inserted through holes provided in anode27, in the supporting tubes 28, and: in the flange portion of anode 26terminate in threaded holes provided around the periphery of the centralaper-VV ture of supporting plate 23, and vsupport the anode assembly ina concentric arrangement.

The anode structure including cylindrical anode 24,'

focusing anode 26, circular anode 27, supporting tubes 28, and also theelectrode shield 36 are preferably constructed of graphite.

electron bombardment.

Integral with supporting plate 23 and coaxial withr to, aretwoelectrically conducting electrode supports 33` and 34. These supportsextend into the ionlzation cham-- The ber and support a circular planarelectrode 35. electrode or filament 35 is connected to the supports 33and 34 at two diametrically opposed points, and 1s located Graphite ischosen because of its` high melting point, and low X-ray productionundery n'a plane intermediate the anodes` 26 and 27,lthus transverselybisecting the cylindrical volume dened'by the radially tapering edges ofthese two anodes.

.Electrode supports 33 and 34 also support a graphite electrode shield36, which consists of a circular channel forming member having thechannel forming edges extending toward itscenter. The upper surface ofthe cylindrical area defined by these channel forming edges is coplanarwith the annular lower surface of anode 26, while the lower surface ofthis cylindrical area is coplanar with the annular upper surface ofanode V27. The inwardly extending extremity of the upper channel formingmember is of greater radius than the peripheral surfaceof anode 26, andis thus electrically insulated therefrom. The lower channel formingmember of shield 36 is similarly spaced from anode 27. 'The filament V35is located concentric with, and transversely bisecting the cylindricalarea deiined'by the shield 36. Thus, the filament is virtually .enclosedby the anodes26 and 27 andthe :shield 36. The shield 36 `is operated atthe same high negative potential as y thefilarnent, -andis ielectrically connected and supported by `electrode support 33. However,in order that the filament heating .voltage will not'be shortedthroughtne shield 36, the1 shield is'supported by, but is electricallyinsulated from, Vsupport post 34.

`The purpose of this arrangement, ie., thefen'closing ofthe filamentand-thehigh negative voltage impressed upon filament shield Y36 will bediscussed hereinafteriin the-operation of the device.

Theouter surfaces of cases 29 and'I01aresurrounded by awater containingcooling jacket l7-to-maintainthe assembly at workable temperatures.

VThe compactness and portability of the present device results fromtheutilization of a short facceleratinggap. This one factor results inthe removal of theformerly encountered obstacles of gaseous dischargeand pressure differentials within a device.

fIn eliminating pressure differentials it has'beeni pointed out abovethat the length of the acceleratinggap'determines thepressuredifferential required in theprior art devices. The present inventionmakes use of accelerating `gaps sufficiently shortzso that'nopressuredifferential isrequired, and has the further effect of allowinghighergas pressures to be used throughout the device. -Furthermore, althoughshort accelerating :gaps -are used, their length with relation-to theaccelerating voltageand .gas pressure is determined in such amannerthat-nofgaseous discharge takes place. This result `isfattained`by first considering the Vfundamental characteristics of a voltagedischarge in a gas.

According to the ,general theory of lsparkingpotentials in gases, andwith particular 'reference to Paschens law, it is well recognized thatthe .electricalpotential required to initiate a discharge through a gasfrom the high voltage electrode to a grounded electrode is a function ofboth the `gas `pressure and distance between the electrode and ground(see Loeb, Fundamental Processes of Electrical Discharge in Gases(Wiley'and Sons, 19'3-9) Vpages 408- 4l-4). Thus, for a particulargas, acurve maybe obtained in which the high voltage, commonly referred to assparking potential, is plotted as the ordinate, and in which the productof pressure and distance between the electrode and ground is plotted asthe abscissa. Figure 3 shows Va representative curve Vof this type.

The straight portion of the curve below the minimum which extendsupwardly to the left follows to a good degree of approximation theequation,

where Vs is equal to the sparking potential in kv., p is thepressure inmm. of Hg, d is the distance betweenelectrodes, i.e., the distancebetween the high voltage elements, such as 20 and 22, and ygroundedelements such as 10, 24 and 26 in the present invention, and `A `and Bare-,constants which are functions ofthe characteristics of the gas. Thevalues of A and B are determined V'by routine experiments in accordancewith ordinary procedures. See Quinn, Physical Review, 55, 482 (1939) for`examples of these procedures. See also Loeb, supra, pages 476 to 484.

Therefore, after routinely determining the values of A and B for theparticular gas and electrodes to be used, a curve is obtained from whichthere can be determined the various combinations of VS and pd at which agaseous discharge will occur. Once an accelerating voltage range hasbeen selected, either the-distance or the pressure may be varied in sucha manner that their product is less than the -value required to initiatea discharge. This product would be any value which would fall to theleft of the curve defined by VS=A -B log (pd) at the acceleratingvoltage used. lBy selecting a voltage a horizontal line (a) is definedas shown in Figure 3. The portion of this line lying to the left of thecurve defines various products of pressure distance values at which lnogaseous discharge will occur. Therefore, by selecting a gas pressure ofl0 to 20 microns, for example, the distance from the high-voltage toground is easily determined. In the present device a distance of l cm.was found to be less than the value required to satisfy thepressure-distance product which would give a discharge at the selectedvoltage. Another factor to consider in the selection of the gap lengthis the mean free path of the accelerated ions. The gap length should besome small fraction of this mean free path, so that ion beam losses arevery low. Thus, in the present device operating at a pressure of abouttwo microns, 8O percent of the ions will have traveled five centimeterswithout making a collision. The gap length used is only one-fifth thatdistance. IOnce this distance is determined, all elements maintained athigh potential, such as elements 20 and 22, are located within thispredetermined distance from the grounded elements, such as case 10 andanodes 24 and 26. IIt is readily apparent, therefore, that thesuppressor envelope 22 functions to reduce the distance between the highvoltage target 20 and support tube 15, and the surrounding groundedelements, thereby increasing the maximum allowable pressure that can beused before a gaseous discharge takes place, as well as to suppresssecondary electrons emitted from the ion bombarded target.

Since the electric field created between the high negative potentialenvelope 22 and the case 10 and anode 24 `will be essentiallyperpendicular to all surfaces of the grounded elements surrounding theenvelope 22, there is no problem of creating a spurious gaseousdischarge between, for example, the upper surface of anode 26 and theextreme upper portion of the envelope 22. As noted above, the upperportion of the envelope 22 is held irmly'against the bottom of insulator11. The purpose of this contact is to reduce the perpendicular distancefrom the case 10 to the high voltage elements 13 and 15, so that thisdistance will be less than the distance required to initiate a highvoltage breakdown, as dened in the above equation.

Electrical circuit The target 20 is connected through target backingelement 18 and target support tube 15 to the negative side of anystandard kv. direct current power source (see Figure 4). The positiveside of this power source is grounded to the case 10, which grounds theanode 24, focusing anode 26, and circular anode 27. The target 20 beingat a high negative potential will attract all positively chargedparticles and repel all negatively charged particles. The suppressor 22is also connected to the negative terminal of the 120 kv. power source,through tube 13. The connections to target 20 and suppressor 22 arearranged in any standard manner so 7 thatuthe, voltage of onemay bevaried independently. of the other.

Anode 27 may also be connected directly tothe grounded positive terminalof the 120 kv. power source, in order to minimize transient electricaleifects between circular anode 27, focusing anode 26 and. cylindricalanode 24.

The electrode 35 and its electrically connected shield electrode 36 aremaintained by means of a 500 v; direct current source at a negativepotential of 500 volts. An electrode heating voltage of 10 volts D.C. isimpressed between the supports 33 and 34. This voltage is not shortedthrough the shield electrode 36, since this electrode is electricallyinsulated at its connection to support 34 by means of an insulator (notshown). Thus, while being maintained at the negative 500 volt potential,the shield 36 does not form a parallel conducting path for the heatingvoltage of electrode 35.

Operation `In operation the device is rst outgassed by baking andpumping out gas emitted from the elements surrounding the internalchambers. The device is then flushed with deuterium gas, so that uponsubsequent operation at elevated temperatures no gas impurities will becontained within the device or emitted from the various internalelements. Deuterium gas is admitted through gas inlet 31 until a gaspressure of from 10 to 20 microns is attained. The inlet 31 is thensealed. This permits the use of the device without the necessity ofburdensome gas inlet leads, gas supplies, vacuum pumps vand vacuum pumpleads, and consequently makes the device easily transportable.

If desired, however, the neutron source maybe operated with a smallpressure difference between the accelerating chamber and the ionizationchamber. This is accomplished by connecting the pumping leads 25 and 32to independent vacuum pumps which evacuate the respective chambers atdifferent rates. See The Review of Scientific Instruments, vol. 24, No.6, pages 426-427, June 1953, for a discussion of this type of operation.Y

With filament heating voltage of l v., the negative electrode potentialof 500 V. D.C., and the target 20 and suppressor 22 high voltage ofnegative 80 kv. D.C :impressed on the respective elements of the device,electrons are emitted from the filament 35. These electrons are forcedaway from the filament, since it is maintained at a negative 500 voltsD.C., and toward the grounded anodes 26 and 27. The shield electrode 36being at the same negative voltage as the filament `also functions toconstrict the emitted electrons to the area at the center of thecircular filament 3S.` This arrangement permits the use of a shortpositive ion accelerating gap, yet allows the ionizing electrons to havea fairly long path to increase the probability of collision andresulting ionization. Ionization of the sourrounding deuterium gasoccurs in the area enclosed by shield 36, circular anode 27, andfocusing anode 26. Thus, the ymajor portion of the ionization takesplace adjacent the aperture in focusing anode 26. The formed ions, beingpositive particles, are influenced by the presence of the negativepotential on the target 20, and are yattracted in that direction. Theaccelerating voltage functions as a focusing voltage, and thepenetration of the electrical field of this voltage functions to pullthe ions toward and through the central aperture in the focusing anodeinthe form of a beam, which is accelerated against the target 20.

The positive ions are accelerated by the high negative voltage of thetarget 20 and bombard the target. The bombarding of the hydrogen isotopewhich is absorbed in the target material results in the liberation ofneutrons according to one of the reactions,

The first of these reactions involving the bombardment of a tritiumcontaining target is the preferred embodiment, since the energy andnumber of the released neu# trons is considerably higher than thatobtained bythe second reaction.

The present device can also be operated with the target at groundpotential and the chamber at a high positive Voltage. The potentialdilference' between the anode 26 and the target 20 would be the same asthat in the preferred embodiment.

As a result of the higher gas pressures used in the present device,i.e., lO to 20 microns, the ion densitiesv in the ionization region areincreased over those of prior art devices. As a result of this large iondensity and the low ion losses over the short accelerating gap, the ion.beam current to the target is approximately onev milliamp. This is anincrease over prior art devices by. a factor of from ten to a hundred.

The larger beam current, however, reduces to some extent the targetlife, since the focused ion beam strikes the target at a spot with adiameter of less than 1 mm. The ion beam spot diameter may be increasedby the following additional apparatus. A tube 38 (see Figure 5),'preferably platinum, about 1A; inch in diameter and having thin walls isinserted in a centrally located hole in the circular anode 27. The tube38 extends upwardly through the aperture in focusing anode 26, andterminates at the upper face of anode 26. This tube functions as a guidefor the positive ions being accelerated toward the target, and resultsin a spreading of the ion beam target spot to a value of about 6 mm. Thespot using this apparatus has its greatest intensity near the center andalong the circumference.

The presence of the tube 38 also has the effect of focusing secondaryelectrons escaping from target into a beam less than 1,46 inch indiameter. These electrons proceed down the axis of the tube 38 andstrike the circular anode 27. In order to prevent secondary electronpenetration of the anode 27, a diamond 39 or other similar hardsubstance is provided at the lower extremity of the tube 38. Theelectrons proceeding down the tube strike the diamond 39. The electronsstriking the diamond, however, will drill a hole through the diamondafter extensive use of this mechanism. Thus, this apparatus, whileeffectively increasing the ion impact spot.

ing table shows the yield for different operating conditions, for the D,T reaction.

Target Pressure, Neutrons Acc. Volt., kv. Current, Microns per sec. ma.(x im) The accelerating voltage'range of from 55 kv. to 97 kv. gives arange of target currents varying from .5 to 7.0 ma. for pressuresranging from 5 to 17 microns.A The neutron yields for these ranges varyfrom .2X 108 to 2.2 X 108 neutrons per second. It should be noted thatthe relationship between neutron yield and any other variable' is notconsistent or predictable. This lack of a definite relationshipapparently indicates that for certain combinations of variables an ionspecies is produced which is less favorable to neutron production. Oneexplanation i* il f g,

of this is that theratio of D+ ions to D2+ ions, i.e.,. the ratio ofatomic ions to molecular ions, varies for different operatingconditions, resulting :in kunpredictable r fluctuations in the targetcurrent. The molecular ions'havetthe effect of reducing the neutronyield, since their mass is greater and, therefore, their accelerationvis less. Molecular ions are formed `by the combination of atomic ionsupon impact with metal surfaces. Thus, the production of molecular ionscan be considerably reduced by..coating the inner metal surface of thedevice with a ceramicmaterial. Although such a coating will to a certaindegree reduce the ruggedness of the present device, the neutron yieldshould be effectively increased.

The yields of the present neutron source are, however at least 108neutrons per second in all cases, representing an increase in yield by afactor of from 100 to 1000 over prior art devices.

What is claimed is:

1. In a neutron source the combination of an electrically groundedchamber, deuterium gas in said chamber, means located at one end of saidchamber for forming ions in said gas, a target containing a hydrogenisotope having at least one neutron supported in opposed relation tosaid ion forming means, means for impressing a high negative potentialon said target for attracting ions formed in said ion forming means, ionfocusing means including an electrically grounded anode located betweensaid ion forming means and said target for focusing into a beam ionsattracted to said target, an envelope having open ends supportedadjacent, electrically insulated from, and enclosing said target, saidtarget being supported within said envelope and adjacent one of saidopen ends, means for impressing a high negative potential on saidenvelope for suppressing electrons emitted from said target, said targetand said envelope located with respect to said electrically groundedchamber and anode so that the distance between the surfaces of said highvoltage target and envelope and the adjacent surfaces of saidelectrically grounded anode and chamber is substantially constant,

2. In a neutron source the combination of a chamber; deuterium gas insaid chamber; means located at one end of said chamber for forming ionsin said gas; high voltage target means supported in opposed relation tosaid ion forming means for attracting ions formed in said ion formingmeans; focusing means including an anode located between said ionforming means and said high voltage target means for focusing into abeam ions attracted to said target means; high voltage open en dedenvelope means located adjacent said target means for suppressingsecondary electrons emitted from said target means, said target meansand said envelope means being located with respect to said anode andsaid chamber at a distance less than the distance defined by theequation:

VB=AB log (pd) where x,=target voltage A and B are constants p=deuteriumgas pressure in chamber d=distance.

3. The neutron source of claim 2, wherein said first named meansincludes a circular planar electron emitting filament, a filament shieldlocated adjacent the periphery of said filament, and negative highvoltage means connected to said shield for restricting said electrons tothe area defined by said filament.

4. The neutron source of claim 2, wherein said focusing means includes acentrally apertured anode located with respect to said target so thatthe field created by said target high voltage means penetrates into saidion forming means through said anode aperture.

5. In a neutron source the combination of an electrically groundedchamber, deuterium gas in said chamber; said chamber containing ionforming means, ion focusing means including an electrically groundedanode,

'gto

anda target in opposed Irelation to said ion "forming means;high'voltagemeansconnected to said'targetA for accelerating said ions toward saidtarget; -said anodebeing located `between said ion forming means andsaid target; open .ended envelope meansconnectedto saidhighflvoltage`means and insulated from said target for suppressing secondaryelectrons emitted from said target, said envelope means and said targetbeing located with respect to"said chamber and said anode at a distanceless than the distance defined by the equation: target `vlt age=AB log(deuterium gas pressure X distance), where A and B are constants;

6. In a neutron source the combination of an electrically groundedchamber; a deuterium gas pressure in said chamber; means located at oneend of said chamber for forming ions in said gas; a target supported atand electrically insulated from the other end of said chamber; saidtarget having one surface containing a hydrogen isotope having at leastone neutron in opposed relation to said ion forming means; high voltagemeans electrically connected to said target for accelerating said ionsaway from said ion forming means and against said target; meansincluding an anode for focusing into a beam said ions accelerated awayfrom said ion forming means, said anode being located between andconcentric about a line connecting said ion forming means and saidtarget, said anode being electrically grounded; open ended envelopemeans electrically connected to said high voltage means and surrounding,and electrically insulated from, said target for suppressing electronsemitted from said target, said envelope means being located with saidopen ends concentric about said line; said envelope means and saidtarget being located with respect to said electrically grounded chamberand focusing means at a distance less than the distance defined by theequation: target voltage=A -B log (deuterium gas pressure X distance),where A and B are constants.

7. In a neutron source the combination of an electrically groundedchamber, deuterium gas in said chamber; a circular planar electronemitting cathode supported proximate the end of said chamber; meanssurrounding said cathode for confining electrons emitted from saidcathode within the area defined by said circular cathode; a targetsupported in said chamber in opposed relation to said cathode; anelectrically grounded anode having a central aperture, said anodesupported intermediate said filament and said target; high voltagemeans; means connecting said high voltage to said target foraccelerating ions, formed by the bombardment of said gas by saidelectrons, through said anode aperture and against said target; openended envelope means supported adjacent said target and connected tosaid high voltage means for suppressing secondary electrons emitted fromsaid target; said target and said envelope means being located withrespect to said electrically grounded chamber and anode a distance lessthan the distance required to initiate a gaseous discharge of said highvoltage on said target and said envelope means to said ground, wheresaid last named distance is defined in the equation: target voltage=A-Blog (deuterium gas pressure X distance), where A and B are constants.

8. In a neutron source the combination of an electrically groundedchamber; deuterium gas in said charnber; a circular electron emittingfilament supported proximate the end of said chamber; means for focusingelectrons emitted from said filament, a first electrically groundedapertured anode; a second electrically grounded anode; a targetsupported in said chamber in opposed relation to said filament; said rstanode supported between said filament and first named means and saidtarget; said second anode supported adjacent said filament and on theopposite side of said filament from said target; said target containinga hydrogen isotope having at least one neutron; means for impressing ahigh negative potential on said target for accelerating ions, formed byequation:

Y 11 the bombardment of said gas by said electrons, against saidYtarget; open ended envelope means supported adjacent said target forsuppressing secondary electrons emitted from' said target; said targetand said envelope means being located with respect to said chamber andsaid rst anode at a distance less than the distance dened by th Targetvo1tage=AB log (pd) where A and B are constants p=deuterium gas pressurein said chamber d=distance.

2,211,668 2,240,914 'Schutze May 6, 1941 2,287,619 Kallmann et al. June23, 1942 2,576,600

v Hanson Nov. 27, 1951:

